Project Summary Sedentary behaviors (i.e., sitting/lying with low energy expenditure while awake) increase risk for cancer, cardiovascular disease, and mortality, particularly among those who do not engage in recommended levels of moderate-vigorous physical activity (MVPA). Sedentary screen time (SST; i.e., television viewing and/or video streaming outside of work and educational pursuits) consumes more than half of available discretionary time and is the single most prevalent use of time for Americans outside of work and sleep. Efficacious strategies to reduce sedentary time in other contexts (i.e., workplace) have been developed by our team and others. SST poses far greater health risks than other sedentary pursuits, yet just two intervention studies have exclusively focused on reducing SST. Moreover, the amount and context in which SST is accrued has dramatically shifted since these studies were conducted. Adults (23-64 years of age) only spend half of their SST viewing a traditional television - the other half is spent on mobile streaming platforms (e.g., Netflix) that are likely to promote ?binge-watching.? There is a need for innovative approaches to reduce SST among adults that are responsive to current and emerging technologies. We will leverage our existing theory-driven and empirically- based mHealth platform, which focuses on reducing total sedentary time in adults, to test StandUPTV, an app that will now specifically target SST. The core StandUPTV intervention will include passive and objective self- monitoring of SST and a behavioral target to reduce SST by 50%. Based on our preliminary data and prior research, we have identified three distinct behavioral components that we will add to StandUPTV: (1) electronically prohibiting SST after weekly 50% limit is reached; (2) adaptive text-messaging based on most recent SST bout; and (3) earning 30 min of additional SST with 10 min bout of MVPA. We will use a multiphase optimization strategy (MOST) to conduct a 16-week factorial experiment to identify which component(s) from the three components under consideration for inclusion in StandUPTV reduce SST by >60 min/day in adults (23-64 years of age) with overweight and obesity. We will also examine how light-intensity physical activity (LPA), MVPA, other factors (e.g., dietary intake), and markers of disease risk (e.g., body weight) change when SST is reduced. We will examine moderators (e.g., age, sex, race/ethnicity) and mediators of behavior change (i.e., social cognitive constructs). Innovative aspects of this proposal are the use of state-of-the-art mHealth design to design a highly-scalable intervention that is responsive to current trends in media use, the MOST framework to efficiently identify efficacious component(s), and leveraging reductions in SST as opportunities to increase LPA, MVPA, and related health behaviors. This study will directly lead to an optimized intervention ? one that is most effective and efficient ? to be tested in a longer-term RCT to determine the causal role of SST in lowering chronic disease risk. These findings will produce new targets for health promotion and impact US public health guidelines, which currently do not include explicit recommendations for lowering SST.